1. Field of Invention
The invention relates to a method and a device for producing a circularly cylindrical body consisting of plastics material, particularly a sintered metal blank, which has at least one helical internal recess extending in the interior of the body.
2. Prior Art
Such bodies are needed particularly in the production of drilling tools or drilling tool inserts of hard metal or ceramic materials. Through the helical course of the at least one internal recess, which serves in the finished drilling tool for the feed of coolant or lubricant to the cutting zone, the drilling tool can be furnished with helical cutting grooves, which is often of advantage for providing favourable cutting and machining characteristics and accordingly is desired.
It has already been previously attempted to produce such sintered metal or ceramic blanks by an extrusion process in that the material consisting of sintered metal powder or ceramic powder and binder is forced through an extrusion die which has a cross-section corresponding with the desired blank cross-section and at least one internally disposed core in the form of a pin, which during extrusion of the plasticised material serves for formation of the internal recess extending through the entire blank.
The material issuing from the extrusion die is usually very pressure-sensitive, i.e. the issuing blank deforms extremely easily in the case of external application of force. Since such deformations are no longer reversible and thus lead to blanks which are unusable at least in sections, it has been attempted to further develop the extrusion process so that the blank has helically extending cooling channels already at the time of issue from the extrusion die. According to one proposal (see, for example EP-A-0 465 946), this is achieved in that formed at the inner circumference of the extrusion die are helically extending guide strips which impose a twisting motion on the issuing plastic material. Flexible threads with a cross-section corresponding with the cross-section of the internal recess to be produced are fastened in the cross-section of the extrusion nozzle, wherein the threads extend up to the outlet of the die mouthpiece. Due to the flexibility of the threads these can follow the torsional movement or torsional flow of the plastic material and thus produce at least one internally disposed cooling channel in the blank.
According to a further proposal the die mouthpiece and/or a hub of propeller-like form, to which are fastened the afore-mentioned threads which are flexible or slack with respect to bending, is or are set into rotational movement during the extrusion process, whereby in turn an externally smooth blank with internally disposed helical channels or recesses can be produced.
In the manufacture of such tool blanks it is important for the angle of inclination of the at least one helical internal recess to be kept constant and within closely toleranced limits over the entire length of the blank. This is required because regular cutting grooves are ground into the tool blank after the sintering process. This grinding is carried out by largely automated machines so that in the case of inaccurate production of the helical internal recesses an uncontrollably high wastage rate can result. In that case it is to be taken into consideration that use is made of tools with fully hard metal cutting parts for the reason, inter alia, that utilisation is to be made of the high capability of loading the material, particularly the torsional stiffness. In order to ensure this, the internal recess must not reach too closely to the cutting groove, which in the case of inaccurate production of the helical internal recess cannot, however, be effectively excluded.
In the case of the afore-described approaches for producing the blank with internally disposed helical recesses it is accordingly necessary to monitor the extrusion tool and/or the sintering devices for the extrusion worm or—in present—for the twist-generating bodies during the extrusion process as accurately as possible and adapt to the mass throughput. This has the consequence that comparatively lengthy re-equipping and adjusting times are required at the extrusion tool, with the result that conventional methods are primarily used economically for large batches. For small batches or for the production of drilling tools with larger nominal diameters disproportionately high machine set-up costs arise, whereby the economics of the production method are called into question.
A method and a device for producing a sintered metal blank with internally disposed helical recesses are already known from EP-B1-1 230 046. According to this known method, initially a substantially circularly cylindrical body with at least one internal recess extending rectilinearly in its interior is produced, for example extruded. This body is cut to a desired length and subsequently subjected, while being supported over its entire length on a support, to a rolling movement by means of a friction surface arrangement, the speed of which changes linearly and constantly over the length of the body, whereby the body is uniformly twisted. This twisting is carried out with use of an axis of rotation which intersects the longitudinal axis of the body.
By means of the method known from BP-B1-1 230 046 it is possible to produce sintered metal blanks in which the angle of inclination of the at least one helical internal recess is constant over the entire length of the blank and is kept within closely toleranced limits. As a result it can usually be ensured that the at least one internal recess does not reach too closely to the cutting groove which still has to be formed.
In practice there are increasingly higher demands on keeping the angle of inclination of the at least one helical internal recess over the entire length of the blank constantly within closely toleranced limits.